As the background of this invention, the prior art is illustrated in FIGS. 9 and 10. A tapper body 1 has at its rear end a shank 2 and a flange-shaped fitting 3 which is to be gripped by a manipulator. The shank 2 is to be connected to a driving shaft (not shown) of a machine tool. A large-diameter axial bore 4 is formed in the tapper body 1, and a driven shaft 5 fits in the bore to be slidable within the tapper body.
An engager 6 has a square bore 6a shaped to fit on a reduced size square 5a formed integral with the back end of the driven shaft 5. The engager 6 protrudes radially and is axially displaceable within the bore. A forward clutch 7 is disposed in contact with the periphery of the axial bore and protrudes thereinto, to engage the engager 6 when it is to be rotated normally forward. A neutral or free space 8 is provided in which the forward clutch 7 is not engageable with the engager 6, axially adjacent to and outwardly from the shank 2 in front of the clutch. A reverse clutch 9 is disposed also in contact with the periphery of the axial bore in front of or outwardly of the neutral space 8. The reverse clutch also protrudes into the bore to engage with the engager 6 when it has to rotate in the reverse direction. A small buffer spring 10 supports the reverse clutch 9 so that it can be rotated together with the tapper body 1 and to a slight extent remain axially displaceable.
A seat 11 selectively engaging an inner end surface 5b of the driven shaft 5 is within the axial bore 4 of the tapper body 1. An absorbing spring 12 functioning as a compression spring, is interposed between the seat 11 and the bottom of the axial bore 4 for absorbing any positioning error in the axial direction between an axially driving member in the machine tool and the tapper attached thereto and driven thereby. A recovery spring 14 functioning as a tension spring, is interposed between the forward end of the axial bore 4 and the engager 6 protruding radially and outwardly of the driven shaft 5. The recovery spring 14 urges the driven shaft toward its back end. A coupling 15 is at the forward end of the driven shaft 5, for enabling the selective attachment of a tap holder 16 carrying a tap 13 to the driven shaft.
In use of such a tapper mechanism, the flange-shaped fitting 3 protruding from the tapper body 1 is gripped by the manipulator, and the shank 2 is connected to the driving shaft of the machine tool for rotating the tapper body 1.
As the tapper body rotates, the engager 6 engages the forward clutch 7 so that the driven shaft 5 rotates forward and the tap 13 in the forward end of the shaft cuts a female screw thread into a workpiece. During this process, the driven shaft 5 is guided by and along the bore of the workpiece in which a thread is being tapped. Therefore, the driven shaft 5 advances forward by itself independently of the tapper body 1. When the thread in the bore in the workpiece is completed, the engager 6 disengages from the forward clutch 7 and enters the neutral space 8. Thus, a torque which was transmitted from the tapper body 1 to the driven shaft is intercepted to bring the shaft into its idle position. Next the drive shaft rotates the tapper body 1 in the reverse direction and thereby retracts it axially to a slight extent. As a result, the engager 6 leaves the neutral space 8 and comes into engagement with the reverse clutch 9. The driven shaft is now rotated in the reverse direction and the tap 13 is guided by and along the tapped bore in the workpiece, until it is completely withdrawn from the tapped bore.
FIG. 10 shows another known apparatus of an almost identical structure to that in FIG. 9 by using the same reference numerals for the same parts. A switch-over mechanism 17 is incorporated in the tapper body shown in FIG. 10 so that its reverse clutch 18 always rotates in reverse direction. Therefore, in contrast to the embodiment shown in FIG. 9, in this embodiment the driving shaft in the machine tool need not be switched over into reverse rotation.
A cover 19 is supported on an outer periphery of the tapper body 1, with bearings 20 separating the covers from the body. The cover 19 is maintained in its position by an anchor arm 21. A bevel gear 22 is journaled on a stud shaft secured to an inner periphery of the cover 19. The stud shaft extends perpendicularly to the axis of tapper body. The bevel gear 22 is meshed with a pair of geared portions 24 and 25. One of these portions 24 is integral with a forward clutch 23, while the other geared portion 25 is fixed to the reverse clutch 18. Therefore, the reverse clutch 18 always rotates in the opposite direction than the direction of rotation of the forward clutch 23. The driving shaft of this prior art tapper embodiment need not be reversed and, therefore, it can be more easily controlled.
In both of these prior art tapper embodiments shown in FIGS. 9 and 10, the engager 6 is in releasable engagement with the respective forward clutch 7 or 23, and travels axially together with the driven shaft 5. This structure often causes a problem when the engager 6, and thus, also the driven shaft 5, is changed from its position connected to the tapper body 1 by the forward clutch 7 or 23, to its idle or neutral position. This problem prevents the instantaneous disengagement of the engager 6 from the forward clutch and, thus causes a chattering between the clutch and the engager which is being disengaged therefrom. Such a chattering causes the premature wearing away and abrasion of these parts, and a reduced accuracy of the thread tapping activity. The chattering is repeated by the mating members when engaging with and disengaging from each other within short time periods. The tappers are usually installed in automated machine tools, and a predetermined distance is preset in them as an invariable machining stroke of the tapper body 1. The chattering thus impairs machining accuracy.